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Full text of "Practical Organic Chemistry"

268

PRACTICAL ORGANIC CHEMISTRY

hydrazobenzene (Prep. 51, p.  148) and aminoazobenzene from
diazoaminobenzene (Prep. 70, p. 172),

PREPARATION 39,

Thiooarbamide.—This is an example of a reversible
reaction, in which either ammonium-thiocyanate or thiourea
when heated yields the same equilibrium mixture. It may he
shown by melting a little thiourea for a minute, when the
presence of thiocyanate is indicated by the addition, of FeCLt.

PREPARATION 41.

Alloxan.—The decomposition of uric acid into alloxan and
area renders the constitution of alloxan of value in elucidating
the structure of uric acid. The constitution is derived from the
following facts : Alloxan is decomposed with caustic soda or
potash into mesoxalic acid and urea, and with hydroxylamine it
combines to form violuric acid, which points to the presence of
a ketone group (Baeyer). Barbituric acid and nitrous acid
also give violuric acid, and seeing that barbituric acid has
been synthesised from malonic acid and urea by the action of
phosphorus oxychloride (Grimaux), it is unquestionably malonyl
urea. The relationship of these substances must therefore be
represented as follows :

NH—CO            NH-CO

CO    CO

I         1
NH—CO

Alloxan.

CO     C:NOH

I         I

NH—CO

Violuric acid.

NH—CO

CO     Clio
I          I

NH—CO

Barbituric acid.

A. renewed interest attaches to alloxan since E. Fischer's dis-
covery of the new synthesis of uric acid. The steps in the
synthesis are briefly the following. Alloxan and ammonium
sulphite form thionuric acid, which is decomposed by hydro-
chloric or sulphuric acid into uramil.

NH—CO                        NH—CO

I         I /NH2                 |         |

:o  ->    co   c(          ->    co   CH.NH.,

II        II XSO,H        |         |

NH—CO            NH-CO                        NH—CO

Alloxan.                 Thionuric acid.                         Uramil.

Nil—CO
CO     0

ll